diff --git a/spatialpy/core/reaction.py b/spatialpy/core/reaction.py
index e8591195..7d5f5dff 100644
--- a/spatialpy/core/reaction.py
+++ b/spatialpy/core/reaction.py
@@ -100,18 +100,20 @@ def __init__(self, name=None, reactants=None, products=None, propensity_function
         if reactants is not None:
             for r in reactants:
                 rtype = type(r).__name__
-                if rtype == 'Species':
-                    self.reactants[r.name] = reactants[r]
+                name = r.name if rtype == 'Species' else r
+                if name in self.reactants:
+                    self.reactants[name] += reactants[r]
                 else:
-                    self.reactants[r] = reactants[r]
-
+                    self.reactants[name] = reactants[r]
+        
         if products is not None:
             for p in products:
-                rtype = type(p).__name__
-                if rtype == 'Species':
-                    self.products[p.name] = products[p]
+                ptype = type(p).__name__
+                name = p.name if ptype == 'Species' else p
+                if name in self.products:
+                    self.products[name] += products[p]
                 else:
-                    self.products[p] = products[p]
+                    self.products[name] = products[p]
 
         if self.marate is not None:
             rtype = type(self.marate).__name__
@@ -343,7 +345,10 @@ def add_product(self, species, stoichiometry):
         except TypeError as err:
             raise ReactionError(f"Failed to validate product. Reason given: {err}") from err
 
-        self.products[name] = stoichiometry
+        if name in self.products:
+            self.products[name] += stoichiometry
+        else:
+            self.products[name] = stoichiometry
 
     def add_reactant(self, species, stoichiometry):
         """
@@ -362,7 +367,10 @@ def add_reactant(self, species, stoichiometry):
         except TypeError as err:
             raise ReactionError(f"Failed to validate reactant. Reason given: {err}") from err
 
-        self.reactants[name] = stoichiometry
+        if name in self.reactants:
+            self.reactants[name] += stoichiometry
+        else:
+            self.reactants[name] = stoichiometry
         if self.massaction and self.type == "mass-action":
             self._create_mass_action()